This invention is directed to a multi-stage amplifier and particularly to an amplifier employing ferrite directional couplers between stages to provide negative feedback.
Many conventional amplifier circuits employ a plurality of stages, each stage including an amplifier device such as a transistor or an operational amplifier packaged as an integrated circuit. In order to provide uniform gain and input impedance over a wide range of frequencies, an amplifier stage employing a transistor or operational amplifier frequently includes circuitry for providing negative feedback. As described in U.S. Pat. No. 4,042,887, titled "BROAD BAND AMPLIFIER", a ferrite directional coupler is employed for coupling negative feedback from the output of a transistor stage to the input.
FIG. 1 shows an example of a prior art one-stage amplifier circuit. A transistor 2 is employed as a common-emitter amplifier. The emitter of the transistor 2 is connected through a resistor 4 to ground. The base of the transistor 2 is connected to receive a signal from a directional coupler 6. The directional coupler 6 is connected to receive an input signal to be amplified from an input 8. A second directional coupler 10 is connected to the collector of the transistor 2. The directional coupler 10 is connected to provide ar amplified signal to an output 12 of the amplifier circuit. A feedback path 14 is connected from the directional coupler 10 to the directional coupler 6 to provide feedback.
A two-stage amplifier for providing increased gain may be provided by connecting two stages as shown in FIG. 1 in series. FIG. 2 shows such a two-stage amplifier. The first stage includes components similar to those of FIG. 1 and numbered the same way. A second stage includes a directional coupler 16 connected to receive an output from the directional coupler 10, a transistor 18 configured in common emitter mode and having a base connected to receive a signal from the directional coupler 16, and a directional coupler 20 connected to the collector of the transistor 18 for receiving an output of the transistor 18 and for providing an amplified signal to the output 12 of the circuit.
The amplifier circuit shown in FIG. 2 has the disadvantage that a relatively large number of components are employed, and a large amount of real estate on a printed circuit board is consumed.
It is therefore an object of the present invention to provide an improved multi-stage amplifier in which the total number of components is reduced.